Based on the first-principles calculations, we investigate the structural, electronic, and magnetic properties of defects in monolayer SnS. We study the formation and migration of vacancies at both Sn- and S-sites. In comparison to the S-site vacancy, our calculations show that creating a vacancy at the Sn-site requires lesser energy, indicating that the vacancy at the Sn-site is more likely to be formed in experiments with energetic particle irradiation. For the Sn-rich (S-rich) environment, the vacancy at the S-site (Sn-site) is more likely to be found than the vacancy at the Sn-site (S-site). Reducing the formation of vacancy clusters, the S vacancy remains at the position where it is created because of the high vacancy migration barrier. Both types of vacancies remain nonmagnetic. To induce magnetism in monolayer SnS, we also study the transition metal (TM = Mn, Fe, and Co) doping at the Sn-site and find a significant influence on the electronic and magnetic properties of monolayer SnS. The doping of TM changes non-magnetic monolayer SnS to magnetic one and keeps it semiconducting. Additionally, long-range ferromagnetic behavior is observed for all the doped system. Hence, doping TM atoms in monolayer SnS could be promising to realize a two-dimensional diluted magnetic semiconductor. More interestingly, all the doped TM configurations show a high spin state, which can be used in nanoscale spintronic applications such as spin-filtering devices.

We study the structural stability of crystalline Na${}_3$ OCl in terms of cohesive energies and phonon spectra through the tilting of Na${}_6$ O octahedra. We prove that the crystal Na${}_3$ OCl can be stabilized through octahedral tilts by comparing 14 tilted structures that are consistent with the Howard and Stokes’ group-theoretical analysis of the octahedral tilting in perovskites. We found that all the 14 tilted structures of Na${}_3$ OCl have lower energies than the cubic Pm$\overline{3}$ m structure by about 11 to 16 meV per five-atom unit cell. The tilt angles along the pseudocubic [100], [010], and [001] directions vary in the range of 2.6${}^{°}$ –9.3${}^{°}$ . While the Pnma and P2${}_1$ /m structures of Na${}_3$ OCl are found as the two most stable ones, only the P2${}_1$ /m phase has stable phonon vibrations with a direct band gap of 3.38 eV at the $\mathrm{\Gamma }$ point.

We theoretically study the H₂S adsorption process on (0001) α-quartz SiO₂ surfaces, which is the preconditioning process for the atomic layer deposition growth of metal sulfide materials. The surface structures of dense and fully hydroxylated (0001) α-quartz SiO₂ are energetically stable, but their reaction with a H₂S molecule is not so active, whereas the cleaved SiO₂ surface is chemically reactive to the dissociative adsorption of a H₂S molecule with an adsorption energy of –3.08 eV/molecule. On the cleaved surface, we confirm that adsorbed H₂S is dissociated into H and H-S fragments, and the energy barrier in this reaction process is computed as 0.042 eV.

The structural phase transition of the high-symmetry cubic phase of antiperovskite $\mathrm{Na}$${}_3$$\mathrm{O}\mathrm{Cl}$ is investigated by computing the phonon band structures of 14 different polymorphs with distinct types of $\mathrm{O}\mathrm{Na}$${}_6$ octahedral tilting. The resulting $P$-$T$ phase diagram shows that, at high temperature and low pressure, the high-symmetry cubic structure with $Pm\overline{3}m$ symmetry is the most stable phase. At low temperature and high pressure, on the other hand, the monoclinic structure with $P{2}_1/m$ symmetry becomes the most stable phase. In between those two, there is a region in the phase diagram where the orthorhombic structure with $Bmmb$ symmetry is the most stable phase. To improve upon the quasiharmonic results, we do additional calculations in the framework of the self-consistent phonon (SCP) theory, including lattice anharmonicity by using cubic and quartic interatomic force constants (IFCs). This is particularly important for the high-symmetry cubic phase. We find that by decreasing the temperature, the frequency of the soft phonon at the $M$ and $R$ symmetry points gradually shifts to lower values. From these results, we can infer that a phase transition occurs around 166–195 K upon soft-mode condensation. Due to the proximity of the soft-mode frequencies at both symmetry points $R$ and $M$, we expect a cubic-to-orthorhombic phase transition to be realized via simultaneous condensation of the two octahedral tilting modes.

We study the electronic band structures and topological properties of nonmagnetic kagome metal $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ when applying in-plane biaxial strains through first-principles calculations. From these calculations, we confirm that the flat band of $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ is located at 250 meV below the Fermi level for pristine $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$, and its position can be raised to the Fermi level by applying a finite in-plane biaxial strain. In this deformation process, we also find that there are two distinct topological phase transitions between trivial and nontrivial topological states at 1.5% and 2.9% strains. Finally, through the calculations of electron-phonon coupling, we examine the possibility of superconductivity mediated by electron-phonon coupling for pristine and strained $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ structures and confirm theoretically that the 5% biaxial strained $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ shows superconductivity. Therefore, biaxial strain in kagome metal $\mathrm{N}{\mathrm{i}}_3\mathrm{Sn}$ triggers the flat band to move toward the Fermi level, thereby inducing nontrivial topology and superconductivity simultaneously.

We explore the variation of interband absorption spectra of GaAs spherical sector quantum dots (QDs) in response to a strong resonant laser, using the renormalized wave function method. Even though a spherical sector QD appears identical to a section cut from a spherical QD, it contains a controllable additional spatial parameter, the apical angle, which results in radically different wave functions and energy levels of particles, and is anticipated to exhibit novel optical properties. The obtained findings reveal that the apical angle of the dot has a considerable effect on the interband absorption spectrum. With the increase in the dot apical angle, a significant redshift of the interband absorption peaks has been identified. Increasing the pump laser detuning and dot radius yields similar results. Especially when a powerful resonant laser with tiny detuning is utilized, a dynamical coupling between electron levels arises, resulting in the formation of new interband absorption peaks. These new peaks and the former ones were similarly influenced by the aforementioned parameters. Furthermore, it is thought that the new peaks, when stimulated by a suitable laser, will produce the entangled states necessary for quantum information.